The explosive growth in mobile electronic devices such as smartphones and tablets creates an increasing need in the art for compact and efficient switching power converters so that users may recharge these devices. A flyback switching power converter is typically provided with a mobile device as its transformer provides a safe isolation from AC household current. It was conventional for flyback converters to include a rectifying diode at the secondary (load) side of the transformer but such rectifying diodes are not power efficient. Thus, synchronous rectification techniques have been developed that replace the rectifying diode with an actively controlled switch such as a field-effect transistor (FET) device (e.g., a metal oxide semiconductor field-effect transistor (MOSFET) device) to improve operating efficiencies by taking advantage of its lower power losses.
Conventional flyback converters with synchronous rectification typically include a synchronous rectifier (SR) controller that controls the synchronous rectifier (SR) switch based on a voltage across the synchronous rectifier switch terminals. When this voltage falls below an on-time threshold following the cycling off of the power switch, the SR controller switches on the SR switch so that power is delivered to the load. During this power delivery, the voltage across the SR switch gradually rises above the on-time threshold voltage until it crosses an off-time threshold voltage. This off-time threshold voltage corresponds to the voltage across the SR switch at the transformer reset time when the secondary winding current has ramped down to zero.
The timing of the on and off states for the SR switch is critical to reduce losses. But the control with regard to the on-time and off-time threshold voltages is problematic because the voltage across the SR switch will have a resonant oscillation at the switch on and off times due to parasitic effects. When the SR switch is switched on following the cycling off of the power switch, this resonant ringing could cause the switch voltage to exceed the off-time threshold voltage such that the SR controller would undesirably switch off the SR switch even though the secondary winding current is still relatively robust (it being well before the transformer reset time). To prevent such an undesirable premature cycling off of the SR switch, it is conventional for the SR controller to apply a minimum on-time period with regard to monitoring the off-time threshold voltage following the cycling on of the SR switch. During this minimum on-time period, the controller does not respond to the SR switch voltage exceeding the off-time threshold voltage.
An analogous minimum off-time period follows the cycling off of the SR switch to prevent the SR controller from responding to a resonant oscillation of the SR switch voltage that causes the SR switch voltage to fall below the on-time threshold voltage. But in contrast to the resonant oscillation that occurs at the SR switch on-time, the resonant oscillation at the SR switch off-time is markedly more robust and prolonged. This robust off-time oscillation of the SR switch voltage complicates the setting of an appropriate duration for the minimum off-time period that may be better appreciated through a consideration of the waveforms shown in FIG. 1 for a power cycle 110 and a power cycle 120 of a primary-side power switch S1. In this example, the voltage across an SR rectifier switch (S2) is a drain-to-source (VD-S) voltage for a metal-oxide semiconductor field-effect transistor (MOSFET). Power switch S1 is switched off in power cycle 110 at a time t0. In response to the cycling off of power switch S1 at time t0, the drain-to-source voltage for the SR switch falls below the on-time threshold voltage. The SR switch is thus switched on while at the same time a timer (S2 Timer MIN TON) is started to time the minimum on-time period. The resulting resonant oscillation of the drain-to-source voltage is relatively minor and quickly damped such that the duration of the minimum on-time period may be relatively short and terminated at a time t3.
In response to the cycling on of the SR switch at time t0, the secondary winding current pulses high and begins to ramp down until it reaches zero at the transformer reset time (time t1). At the same time, the drain-to-source voltage for the SR switch rises above the off-time threshold voltage (S2 OFF Threshold) such that the SR switch is switched off and a minimum off-time timer (S2 Timer MIN TOFF) begins timing the minimum off-time period. The resulting resonant oscillation for the SR switch drain-to-source voltage following time t1 is more pronounced and slower to damp as compared to the damping that occurs at the on time for the SR switch. For power cycle 110, the minimum off-time period has a proper duration that terminates at a time t4 such that the resonant oscillations of the drain-to-source voltage do not cross the on-time threshold voltage following the termination of the minimum off-time period.
But the resonant oscillations following the SR switch off time are more pronounced for a subsequent power cycle 120 of power switch S1. Power cycle 120 thus has a relatively-long ringing period whereas power cycle 110 has a relatively-short ringing period. Due to this more pronounced resonance, the SR switch drain-to-source voltage crosses the on-time threshold voltage at a time 122 following the termination of the minimum off-time period in power cycle 120. As a result, the SR controller cycles the SR switch on despite there being no power pulse to deliver. The result is that the secondary winding current has a slightly negative value during the minimum on-time period following time 122. Upon or just after the termination of this undesirable minimum on-time period, the SR switch drain-to-source voltage again exceeds the off-time threshold voltage such that the SR switch is cycled off for another minimum off-time period. The subsequent resonant oscillation of the SR switch drain-to-source voltage again causes it to exceed the on-time threshold voltage such the SR switch is again cycled on at time 124. Another negative current is induced on the secondary winding until the termination of the current minimum on-time period whereupon the SR switch drain-to-source voltage again exceeds the off-time threshold voltage such the SR switch is opened for another minimum off-time period.
The resulting cycling on and off of the SR switch following the transformer reset time is undesirable for a number of reasons. For example, the negative current excited across the secondary winding wastes power. More fundamentally, the SR switch may be cycled on when the power switch S1 cycles on, which is a severe problem. The prior art setting of the minimum off-time period is thus problematic in that it cannot be set too short or this undesirable cycling of the SR switch occurs, yet it cannot be set too long in that the minimum off-time period would then interfere with the next power switch S1 cycling.
Accordingly, there is a need in the art for improved synchronous rectifier control techniques for switching power converters.